A NAND-type flash memory and a NOR-type flash memory have been widely used as nonvolatile memory devices. In recent years, flash memory devices having advantages of both a NAND-type flash memory and a NOR-type flash memory have been proposed. The demand for a system LSI including flash memory circuits as well as logic circuits has also increased.
The system LSI including flash memory circuits and logic circuits have a complicated device structure. The structure of a memory cell of a flash memory differs from that of a MOS transistor of a logic circuit. The memory cell has a gate structure, which is generally provided with double gate electrodes, while the MOS transistor has single gate electrode.
Moreover, the fabrication process of a flash memory partially differs from that of a logic circuit. For example, the storage characteristics are important for a gate insulating film of a flash memory. On the other hand, the reduction of gate capacitance is necessary for a gate insulating film of a logic circuit. In a system LSI, it is required to satisfy performances of both a flash memory and a logic circuit.
Furthermore, it is also required, in the system LSI, to adjust a fabrication process and a device structure between a flash memory and a logic circuit.
Japanese Patent Publication (Kokai) No. 2002-64157 discloses a device structure and fabrication method of LSI including both a flash memory and a logic circuit. For example, a layered structure of three polycrystalline silicon films is used as two gate electrodes. The impurity doping condition is changed for each of the polycrystalline silicon films.
As mentioned above, a system LSI having more excellent performance can be fabricated by adjusting the device structure and the fabrication process between a flash memory and a logic circuit.
However, further improvement is required for higher speed performance of a logic circuit in a future LSI including a flash memory and a logic circuit.